Step 1:The nucleophilic C of the ylid Wittig reagent adds to the electrophilic
C in the polar carbonyl group, electrons from the C=Op
bond are used to form a s bond to the +ve P
atom. This creates a cyclic intermediate called an oxaphosphetane.

Step 2:Decomposition of the intermediate by breaking the C-P and C-Os bonds leads to the formation of the C=Cp bond of the alkene and triphenyl phosphine
oxide.